This application claims priority under 35 U.S.C. xc2xa7xc2xa7 119 and/or 365 to Appln. No. 00810204.8 filed in Europe on Mar. 10, 2000; the entire content of which is hereby incorporated by reference.
1. Field of the Invention
The invention relates to large-scale electric power transmission networks, and, more particularly, to a method and a device for assessing the stability of an electric power transmission network according to the preamble of claims 1 and 7.
2. Background of the Invention
Electric power transmission and distribution systems or networks comprise high-voltage tie lines for connecting geographically separated regions, and substations for transforming voltages and for switching connections between lines. Power generation and load flow in a network with several substations is controlled by a central energy management system. Measurements of average RMS (root mean square) values of voltages, currents, active power and reactive power flowing in the network are made at various places in the network. The measurements from throughout the network are collected at the energy management system, providing a snapshot of the state of the network. Such a snapshot is updated roughly every 30 seconds. The snapshot gives a static view of the network state, in particular of load flows.
A method for assessing network stability, based on voltage margins, is described in the paper xe2x80x9cUse of local measurements to Estimate Voltage-Stability Marginxe2x80x9d, K. Vu et al., Power Industry Computer Applications (PICA) May 12-16, 1997, IEEE, and in xe2x80x9cVoltage instability predictor (VIP) and its applicationsxe2x80x9d, K. Vu et al., Power Systems Computation Conference (PSCC) June 1999. Both articles are herewith incorporated by reference. These articles describe a xe2x80x9cVoltage Instability Predictorxe2x80x9d (VIP) which measures currents and voltages locally in order to infer a proximity to voltage collapse. The concept of the VIP is shown in FIG. 1. One part of an electric power system is treated as a power source, another part as a load. The power source is represented by its Thxc3xa9venin equivalent 21 with a Thxc3xa9venin impedance ZThev. The load is represented by an apparent load impedance ZAPP. Both the Thxc3xa9venin impedance ZThev and the apparent load impedance ZAPP are estimated from the current and voltage measurements by a VIP device 22. The relation of these impedances indicates how close the power source is to collapsing. The paper xe2x80x9cGrids get smart protection an controlxe2x80x9d, K. Vu et al, IEEE Computer Applications in Power, October 1997, discloses VIP devices reporting their findings such as a proximity to collapse to an energy management system.
U.S. Pat. No. 5,734,586 discloses a system for optimising a steady state in an unbalanced power distribution network having a star-shaped structure. The optimisation starts with data representing the state of the network and determines switching commands for reconfiguring lines and control commands for capacitors and voltage regulators for reactive power compensation.
In all the cited documents, reaction times at a system level are limited by the relatively slow system management functions.
It is therefore an object of the invention to create a method and a device for assessing the stability of an electric power transmission network that offers an improved, dynamic view of the state of the network. A further object of the invention is to enable a fast reaction at a network level to sudden changes in load conditions.
These objects are achieved by a method and a device for assessing the stability of an electric power transmission network according to the claims 1 and 7.
The inventive method assesses the stability of an electric power transmission network, where said network comprises a plurality of substations, buses and lines, and a system protection center. It comprises the steps of
1. measuring phasor data for voltages and currents at a plurality of locations of the network,
2. transmitting said phasor data to said system protection center,
3. transmitting information regarding the state of switches of at least one substation to the system protection center, and
4. the system protection center determining at least one stability margin value of the transmission network.
In this way, detailed real-time information about the state of the network is collected at a system level of the network, which allows a corresponding global analysis of the information.
In a preferred embodiment of the invention, the system protection center determines one or more control commands, based on the phasor data and on the state of switches. The control commands are transmitted to at least one substation and executed there.
The system protection center and method in accordance with the invention provide protection of the power network at the system level, in contrast to existing protection schemes that operate locally and only protect individual transmission lines or substation components such as transformers or generators.
In another preferred embodiment of the invention, the system protection center determines information about a state of the network, e.g. power flow and potential or estimated future limits or bottlenecks, i.e. overload conditions to be expected at a given line or substation. This information is transmitted to the energy management system which then controls power generation and power flow within the network according to the network state information. Since this manner of control uses a prediction of the network""s behaviour, overloading of network components is anticipated and avoided before components of the network exceed their operating limits and have to be shut down, potentially aggravating the condition of the network even further.
Since the inventive method and device utilise phasor data, they rely on real-time information on currents, voltages and power, with a high temporal resolution. This is in contrast to scalar information such as RMS values, which corresponds to an average computed over at least one period and is non-real-time information. Such non-real-time information, as it is used in existing energy management systems, has update intervals around 30 seconds. As a result, a reaction time of the inventive method to detect a potential network instability, to determine countermeasures and to issue corresponding control commands is less than 100 milliseconds and typically less than 50 milliseconds. Such a fast reaction time further reduces the danger of disruptive events caused by an overloaded or unstable network.
In a further preferred embodiment of the invention, the phasor data is associated with timing information obtained from a common time distribution system.
Further preferred embodiments are evident from the dependent patent claims.